The present invention provides a process and apparatus for liquid/liquid extractions by means of microporous membranes. It has been found that the rate of mass transfer in such liquid/liquid extractions is increased by the process and apparatus of the present invention.
In general, a mass transfer process can be expressed as EQU N=K(C.sub.i -C.sub.o)
where N is the flux of a species, i.e., the rate of mass transfer, C.sub.i and C.sub.o represent the concentration of the species at different times, i.e., the driving force, and K represents a resistance to the mass transfer, more often termed the overall mass transfer coefficient.
Liquid/liquid extraction is a unit operation separation process which exploits chemical differences between two liquids to affect a mass transfer of a species from one liquid to another. Equipment typically employed in liquid/liquid extraction comprises mixer-settlers, spray and packed extraction towers, and centrifugal extractors. See generally McCabe, W.L. and J C. Smith, Unit Operations of Chemical Engineering, 3rd Ed. (McGraw-Hill, N.Y. 1976) pp. 465-800. These methods employ intimate liquid/liquid contact. Accordingly, these are problems with emulsion formation as well as the contamination of one liquid with the other, such as by back mixing or flooding. Ibid. at 622-623.
To obviate problems inherent in extraction processes involving intimate liquid mixing, the art has used various membranes which function both to prevent the dispersion of one liquid into the other and to serve as the medium across which the extracted species is transferred from one liquid to the other. See Perry, R.H. and C.H. Chilton, Chemical Enqineers' Handbook, 5th Ed. (McGraw-Hill, N.Y., 1969), pp. 17-34 to 17-43.
More recently, selectively permeable membranes have been used in extraction processes. U.S. Pat. No. 4,268,279 discloses a gas transfer process which utilizes a microporous hollow fiber membrane. The patent is directed to the transfer of a solute in the gaseous phase between a first liquid fluid and a second gaseous or liquid fluid. Accordingly, the membrane employed therein is permeable to gaseous components but impermeable to liquids under operative conditions.
U.S. Pat. No. 4,443,413 discloses a process for separating molybdenum mineral values from tungsten values wherein the molybdenum values are transferred from one liquid solution to another liquid solution across a membrane. The patent employs indirect contact of an organic extractant solution and an aqueous leachate solution across a membrane such that the extractant material is not in direct contact with the aqueous feed.
Kiani, A. et al, Journal of Membrane Science, vol. 20, pp. 125-145 (1984), discloses an extraction process employing a microporous hydrophobic membrane having an immobilized liquid/liquid interface. However, Kiani et al discuss only planar hydrophobic membranes for extracting acetic acid and, moreover, do not discuss the criteria by which a liquid/liquid and membrane system is chosen for an extraction process. The present invention is particularly directed to selecting an appropriate liquid to wet the membrane and thereby increase the mass transfer rate in liquid/liquid extractions, a problem which is not addressed by Kiani et al.
By the present invention, the rate of mass transfer in liquid/liquid extraction processes can unexpectedly be increased by orders of magnitude over the mass transfer rates of prior art extraction processes by the proper selection of the appropriate liquid to wet the microporous membrane. The art has heretofore not recognized how to effectively increase mass transfer rates in liquid/liquid extraction processes by the appropriate selection of the liquid/liquid and membrane system across which a solute species is transported
Accordingly, an object of the present invention is to provide a process for increasing the mass transfer rate in liquid/liquid extractions employing microporous membranes.
Another object of the present invention is to provide a process for selecting a liquid to wet a microporous membrane used in such extraction processes to effectively increase the mass transfer rate thereof.
Yet another object of the present invention is to provide a process and apparatus for extractions having a liquid/liquid interface immobilized at a surface of a wetted microporous membrane
Yet further objects will become apparent to the skilled artisan upon examination of the detailed description of the invention.